The process of electrolytic removal of metals from solutions involves immersing a pair of electrodes in an electrolytic solution containing the metal to be recovered and inducing a voltage across the electrodes, that voltage having sufficient magnitude to cause electrolytic deposition of the desired product.
This process has been used in the recovery of silver from used photographic processing solutions. In the processing of exposed photographic films or papers, various silver salts employed in their manufacture are dissolved in the aqueous fixative or stop solutions as complex silver salts. If the silver content of these processing solutions is allowed to rise above a certain value, their chemical action becomes unsatisfactory, and the solution must be replaced. By removing silver from the solutions during processing, the life of the solutions can be greatly extended thereby reducing the cost of the process. In addition, the reclaimed silver can be used as a source of revenue. Finally, the Environmental Protection Agency (EPA) has provided increasingly strict guidelines for the amount of metals which may remain in a solution at the time of disposal, creating a need for highly efficient removal of silver from solution.
Prior art electrolytic methods for recovering silver from film processing solutions possess a number of drawbacks. One of the drawbacks is contamination of the processing solution. Another is the release of noxious gases, as a result of the decomposition of sulphur ions which consequently may produce hydrogen sulphide gas. The release of hydrogen sulphide gas is a result of attack of the plated silver by the sulphide ions.
Prior systems which have attempted solutions to the above problems include U.S. Pat. No. 3,875,032 issued to Thompson, which shows a system for measuring the concentration of silver in solution using dedicated measuring electrodes which are excited to a fixed voltage. The current is used to control the current applied to the primary plating electrodes. U.S. Pat. No. 3,616,412 issued to Gnage discloses a system for measuring silver concentration by determining the time required for the resistance between two cathodes in the solution to drop below a predetermined value. This value is used to control plating current flow. Both of these systems result in some contamination of the solution and in the release of hydrogen sulphide gas due to the occasional use of high plating voltages.
U.S. Pat. No. 4,612,102 issued to Brimo discloses an electroplating power supply which uses two current set points, one high and one low. When the current at the electrodes exceeds the high set point it is an indication that there is enough silver in solution for electroplating to occur. As plating continues and silver is removed from solution, the current drops, causing the applied voltage to drop when the current falls below the low set point. The circuit continues to monitor current and when the current rises above the low set point, the circuit returns to high voltage to continue plating. The plating process continues until the amperage drops again below the high set point and the control recycles as before. No plating occurs from the time the amperage drops below the high set point and the time the amperage rises above the low set point. During this interval, metal can build up in solution because photographic processes continuously add and remove solution. As a result, valuable silver can be lost, and the removed solution may contain more than the minimum amount of metal permitted for disposal without extra precautions.
The apparatus described in U.S. Pat. No. 4,776,931 of Hardy overcomes some of the sensitivity problems by intermittently sampling the metal content of the solution by applying the plating voltage to the electrodes to determine if the current drawn by the solution exceeds a preset value indicating sufficient metal in solution for plating. When the current drawn by the solution falls below a threshold value, a lower standby voltage is applied, that standby voltage being greater than zero, with the higher plating voltage intermittently applied to determine if sufficient metal has been added to the solution to resume plating. The Hardy system samples at approximately four second increments both during plating and in standby. A certain amount of inaccuracy is involved in this sampling because the analog technology provides a relatively slow response time which, particularly during application of the plating voltage, could result in a sufficient delay to cause damage to the electrodes and/or plated metal before the plating voltage is switched off. Another disadvantage of the Hardy system is that it requires the use of two separate voltage regulators, one for negative output voltage and the other for positive output voltage, which can lead to difficulty in calibration and troubleshooting of the equipment.
It is known, particularly for switching circuits, that digital technology provides quicker response than analog technology. It is therefore desirable to provide quick switching and response capability of digital technology to an intermittent sampling apparatus in an electroplating power supply to provide rapid, accurate switching and adjustment to compensate for changes in the metal content of the solution. It is also desirable to provide an electroplating system which is extremely efficient in its removal of metals from solution such that the treated solution meets the increasingly stringent requirements for disposal as defined by the Environment Protection Agency. It is to such an apparatus and method that the present invention is directed.